1. Field of the Invention
The present invention relates to a vapor-phase thermal cracking process of dicyclopentadiene which can achieve a superior cracking rate and can be operated in a stable manner for a long period of time, and to a simple and industrial process for manufacturing high purity dicyclopentadine suitable as a raw material for the reaction injection molding (hereinafter called RIM).
2. Description of the Prior Art
Hithertofore, dicyclopentadiene have been used, as its industrial usage, as a third component of ethylene-propylene rubbers, as a raw material for unsaturated polyesters, and the like. Generally, dicyclopentadiene have been manufactured by a process of dimerizing C.sub.5 fractions by-produced in thermal cracking of petroleums, typified by naphtha, and the like. In this instance, since the products contained many impurities by-produced in the dimerization reaction, such as cyclic codimers of cyclopentadiene with butadiene, isoprene, piperylene, etc. (vinylnorbornene, isopropenylnorbornene, propenylnorbornene, etc., hereinafter called codimers) and polymerization addition reaction products, such as tricyclopentadiene, and the like, and, further, oxides and the like of these compounds, their purity was about 70-95% by weight (hereinafter called crude dicyclopentadiene).
In recent years, an attention has been given, as a novel usage of dicyclopentadiene, to a method for the manufacture of molded articles of polycyclopentadiene resins with a crosslinked structure by RIM in which the polymerization reaction and the injection molding proceed simultaneously by the use of a ring opening polymerization by a metathesis catalyst comprising a tungsten compound or a molybdenum compound and an organo-aluminum compound.
However, if the crude dicyclopentadiene is used for the RIM the metathesis catalyst is deactivated by impurities such as those mentioned above, failing to induce a sufficient polymerization activity of the catalyst. Even if the crude dicyclopentadiene can be polymerized, the articles made therefrom may be inadequately molded or their physical strength may be reduced. Thus, studies concerning processes for the improving the purity have been ongoing.
Among one of the processes, there is a process for removing the impurities by repeated distillation. The process, however, is not economical due to the requirement of a distillation column with a very high number of theoretical plates because of the proximity in boiling points of the impurities and dicyclopentadiene, the necessity of regulating the recovery rate of dicyclopentadiene at a low level, and the like.
Because of this, a process for the manufacture of high purity dicyclopentadiene by the thermal cracking and redimerization of crude dicyclopentadiene has been proposed. Since the rate of thermal cracking reaction is greater in dicyclopentadiene than in codimer, cyclopentadiene is selectively obtained by the thermal cracking when the both are present together. Furthermore, since the rate of the thermal dimerization of cyclopentadiene overwhelmingly predominates that of the codimerization reaction of cyclopentadiene and other diolefins, the selectivity to dicyclopentadiene is even higher. This process utilizes these differences in the reaction rates.
A process for cracking dicyclopentadiene in the liquid phase in the presence of high boiling point hydrocarbon oils was proposed as a process for the thermal cracking of dicyclopentadiene (U.S. Pat. No. 2,831,904). The process, however, is complicated and uneconomical; cracking at a relatively low temperature, i.e., 200.degree.-300.degree. C., affords only a low yield of cyclopentadiene, and a large amount of polymers, such as dicyclopentadiene and the like, accumulate in hydrocarbon oils, which may clog tubes in the apparatus, leaving a problem of exhaust oil treatments, and the like.
Furthermore, a process for the vapor-phase cracking of dicyclopentadiene at a higher temperature in the presence of gaseous diluents such as steam, nitrogen gas, or the like, in order to promote its rate of cracking, was proposed (U.S. Pat. No. 2,582,920). The same problem of clogging the system by the accumulation of carbonized materials in the thermal cracking unit during a continuous operation in a short period of time was indicated concerning with this process. As a method of overcoming this problem, a cracking process was proposed, in which a feed is introduced after the removal of polymers and tar-like materials, which are the cause of the clogging of tubes, by contacting the feed with super-heated steam, and the cracking is carried out not by the external heating but by the use of heated steam itself (Japanese Patent Publication No. Sho 51 (1976)-29145). The process requires complicated pretreatment steps; it is difficult for the process to produce, by itself, high purity dicyclopentadiene suitable as a raw material for RlM in an industrial scale and in a stable manner.